1. Field of the Invention
The present invention relates to a coaxial two-wheel vehicle including two wheels disposed on the same axis center line, and it particularly relates to a coaxial two-wheel vehicle which can travel freely with people riding thereon.
2. Description of the Related Art
As a coaxial two-wheel vehicle of this kind in related art, there is one described in Patent Reference 1, for example. In Patent Reference 1, there is a description relating to a coaxial two-wheel vehicle including wheels at both ends of the same axle. The coaxial two-wheel vehicle described in Patent Reference 1 is “a coaxial two-wheel vehicle configured to have a pair of wheels; a wheel axle provided between the pair of wheels; a base capable of tilting supported above the wheel axle; a pair of drive motors mounted on the base to drive each of the pair of wheels; and a controller that sends an operation command to the pair of drive motors, wherein an acceleration detector for detecting acceleration in the vertical direction is provided in the base, and the controller sends an operation command to decelerate to each of the pair of drive motors when an absolute value of the acceleration detected during travel by the acceleration detector is a predetermined threshold value or more”.
According to the coaxial two-wheel vehicle of Patent Reference 1 having the above-described configuration, the vehicle is expected to obtain such effectiveness that “in a case, for example, of running onto a step, since the acceleration detector for detecting acceleration in the vertical direction is included and the operation command to decelerate is sent to each of the pair of drive motors when an absolute value of the acceleration detected during travel by the acceleration detector is a predetermined threshold value or more, safe travel can be performed by following the step and the like.”
In addition, as another example of a coaxial two-wheel vehicle in related art, there is one described in Patent Reference 2, for example. In Patent Reference 2, there is a description relating to a method of controlling the posture of a coaxial two-wheel vehicle. The posture control method in the coaxial two-wheel vehicle described in Patent Reference 2 is “in the coaxial two-wheel vehicle configured to have a pair of wheels; a wheel axle provided between the both wheels; a vehicle body supported above the wheel axle in a turnable manner; a wheel drive motor mounted on the vehicle body; a control computer that sends an operation command to the wheel drive motor; and an angle detector for detecting an inclination of the vehicle body, an angle of inclination of the vehicle body detected by the angle detector is sampled at short-time intervals, computation is performed by assigning a sampled value to a control input calculation formula that is input and set in advance in the control computer, where the sampling inclination angle of the vehicle body is used as a state variable and a feed-back gain is used as a coefficient, controlling torque for the wheel drive motor is calculated based on the computation; and a command to perform an operation equivalent to the calculated control torque is sent from the control computer to the wheel drive motor.”
According to the posture control method in the coaxial two-wheel vehicle of Patent Reference 2 that has the above-described configuration, it is expected to have such effectiveness that “in case that the vehicle body tilts, immediately the wheel shifts in the direction of the vehicle body inclined and so a restoration of posture of the vehicle body is performed without fail, because the computation is performed by assigning a sampled value to the control input calculation formula that is input and set in advance in the control computer, where the sampling inclination angle of the vehicle body and the feed-back gain are used as the coefficients; controlling torque for the wheel drive motor is calculated based on the computation; and feed-back control of the wheel drive motor is performed based on the result of this calculation”.    [Patent Reference 1] Japanese Published Patent Application No. 2005-6436    [Patent Reference 2] Japanese Published Patent Application No. S63-305082
However, in the coaxial two-wheel vehicles described in the above-mentioned Patent References 1 and 2, a handle is fixed to a step plate (riding portion) for a man to ride, a support portion supporting the wheels in a freely rotatable manner is fixed to the step plate, and the upper surface of the step plate (riding surface) is continuously in parallel with a traveling surface (road surface). Accordingly, when the center of gravity is at a high position like a standing posture in which a man is riding in a standing state, an upper body of the rider becomes unstable by being swayed right and left due to an action of gravitational force at the time of traveling on a cant road surface where the road surface inclines in the direction orthogonal to a traveling direction or due to an action of centrifugal force at the time of turning, and there is a possibility that the vehicle body is overturned in the lateral direction when the force becomes considerably large.
Details are explained in this regard by referring to FIGS. 1 through 3. FIGS. 1A through 1C are explanatory diagrams respectively showing a state of the coaxial two-wheel vehicle of related art viewed from the front side of the vehicle. In FIGS. 1A through 1C, reference numeral 1 denotes the whole of a coaxial two-wheel vehicle in which a vehicle body 2 used also as a step plate is provided. Left and right wheels 3L and 3R are rotatably provided on both sides in the direction orthogonal to a traveling direction of the vehicle body 2. In addition, reference numeral 4 denotes a riding object (such as a man, for example) riding on the vehicle body 2, reference symbol G denotes the center of gravity of the rider 4, and reference symbol W denotes a weight (load) of the rider 4.
FIG. 1A shows a state of the coaxial two-wheel vehicle 1 traveling straight on a flat road surface without an influence of lateral force and centrifugal force. In this state, the center of gravity G of the rider 4 is positioned approximately above the center of the coaxial two-wheel vehicle 1 and the load W acts vertically to work at the approximate center of the vehicle body 2. Accordingly, approximately the same load acts on the left and right wheels 3L and 3R, and the reaction force thereof becomes approximately the same at ground contact points TL and TR where the wheels 3L and 3R come into contact with a road surface E.
FIG. 1B shows a state of the coaxial two-wheel vehicle 1 making a turn on the flat road surface E. In this state, centrifugal force (lateral force) F acts on the rider 4 from the right wheel 3R side, and a weight vector W of the load W slants by an angle θ due to the influence of the centrifugal force F. When a ground contact point R where an extended line of the weight vector W intersects the road surface E is inside the ground contact point TL of the left wheel 3L, the coaxial two-wheel vehicle 1 can make a turn with stability. However, when the ground contact point R is outside the ground contact point TL, as shown in FIG. 1B, the stability of the traveling is impaired, because the left and right wheels 3L and 3R are unable to bear the centrifugal force F, and the vehicle may overturn (falling in the lateral direction), as shown in FIG. 1C, when the centrifugal force F becomes considerably large.
A difficulty level causing this coaxial two-wheel vehicle 1 to overturn greatly depends on the height of the center of gravity G of the rider 4. FIG. 2 is a diagram to explain about that. When the center of gravity G of the rider 4 is at a low position, a tilt angle allowed to the weight vector W of the center of gravity G is an angle θ as shown in FIG. 2. However, when the center of gravity G of the rider 4 is high and shifts to the center of gravity G1, the tilt angle at the center of gravity G1 becomes an angle θ1 which is smaller than the angle θ (θ1<θ), since a distance S from the center of the vehicle body 2 to the ground contact points TL and TR of the left and right wheel 3L and 3R remains unchanged.
From the above, it is understood that the difficulty level of causing the coaxial two-wheel vehicle 1 to overturn is expressed by a product of the height of the center of gravity G and the centrifugal force F. Specifically, assuming that the ground contact point R of the weight vector W corresponds to the ground contact point TL of the left wheel 3L when the centrifugal force F acts on the center of gravity G, F×H=S (expression 1) can be obtained. Similarly, assuming that the ground contact point R of a weight vector W1 corresponds to the ground contact point TL of the left wheel 3L when a centrifugal force F1 acts on the center of gravity G1, F1×H1=S (expression 2) can be obtained. Accordingly, F×H=F1×H1. Here, F>F1 because H<H1. Therefore, when the center of gravity is positioned higher, the coaxial two-wheel vehicle 1 may overturn, even if the centrifugal force becomes smaller by that much.
Such a overturn of the coaxial two-wheel vehicle 1 can be prevented with a structure shown in FIG. 3. FIG. 3 is a diagram showing the vehicle body 2 being inclined toward the road surface E on the right wheel 3R side where the centrifugal force F acts. When the vehicle body 2 is thus inclined to the side where the centrifugal force F acts, a overturn of the coaxial two-wheel vehicle 1 can be prevented and a stable turning 1 becomes possible, because the ground contact point R of the weight vector W1 shifts to the inside of the ground contact point TL of the left wheel 3L.